Ventricular tachycardia occurs on ambulatory monitoring in 19% of patients with hypertrophic cardiomyopathy (HCM), of whom 8% die yearly. However, high risk subgroups have not been identified. To determine the role of programmed electrical stimulation in defining contributing factors to major clinical events and in directing therapy, we studied 29 HCM patients with cardiac arrest, syncope, near-syncope, or asymptomatic nonsustained ventricular tachycardia. Programmed electrical stimulation identified abnormalities in 15 patients (52%), including inducible ventricular tachycardia in 6 patients, atrial ventricular node disease in 5, bypass tracts in 2 and supraventricular tachycardia in 3. Patients with near syncope or asymptomatic nonsustained ventricular tachycardia did not have inducible tachycardia. However, in 73% of patients with syncope a potential contributing abnormality was found, and 80% of cardiac arrest survivors had inducible ventricular tachycardia. These data indicate: 1) asymptomatic nonsustained ventricular tachycardia and near syncope may represent lower risk subgroups and 2) programmed electrical stimulation frequently identifies potential mechanisms of syncope or cardiac arrest, and therefore may be useful in directing therapeutic strategies.